1. Field of the Invention
The present invention relates to an optical device, an optical module, and a method for manufacturing an optical device.
2. Description of the Related Art
The widespread use of broadband services via the Internet leads to a rapid increase in communication traffic flowing on the network. The capacities of a trunk network between large cities or continents and a metropolitan network connecting adjacent cities are actively increased. Also in a datacom such as a storage network or Ethernet (trademark) as well as a conventional telecom network, the technology for increasing the capacity of a system has become more and more important. The throughput of a high-speed interface device for such network is limited by a speed per channel and also the package density determined by the module size and power consumption. Therefore, the downsizing of a module is an important issue.
For example, in a 10 Gbps optical transceiver module, a more compact module such as a 10-Gigabit small form-factor pluggable (XFP) module or a small form-factor pluggable plus (SFP+) module has become widespread in place of a conventional relatively large 300-pin module. In a tunable transceiver module that operates in the entire C-band region of wavebands from 1,530 nm to 1,570 nm, in place of a 300-pin large-form factor (LFF) module and a 300-pin small-form factor (SFF) module which are now prevailing, the downsizing to a compact XFP module whose volume is about 1/15 is being strongly demanded.
The key technology for downsizing an optical module is an integration technology for optical devices. For example, in a middle and long distance transceiver module for a metropolitan network, a distributed feedback (DFB) laser source in which semiconductor electro-absorption modulators are monolithically-integrated is used on the transmission side, thereby realizing a compact optical transceiver module. This method has already been put into practical use.
The integration technology for optical devices is roughly divided into two types. One is an integration technology from the viewpoint of how to connect core layers of waveguide optical devices together. The other is an integration technology from the viewpoint of how to connect mesa structures of waveguide optical devices together.
Specifically, examples of the integration technology from the viewpoint of how to connect the core layers of the waveguide optical devices together include a butt joint method and a region selective growth method. The butt joint method is a technology for integration by joining a plurality of optical waveguides on the same substrate by butting the optical waveguides. In the process, first, crystals of a core layer of a first waveguide optical device are grown on a semiconductor substrate, and then, a part of the core layer is covered by a mask pattern and the remaining part not covered by the mask pattern is removed with the use of an etching technology. Subsequently, with the use of metal organic chemical vapor deposition, crystals of a core layer of a second waveguide optical device are grown in a region of the core layer of the first waveguide optical device which has been removed by etching, to thereby connect the core layer of the first waveguide optical device and the core layer of the second waveguide optical device to each other. This process is repeated the required number of times. According to this technology, the material compositions, the multilayer structures, and the thicknesses of the respective core layers of the waveguide optical devices can be optimized independently. The butt joint method is therefore widely used as a method of manufacturing a high-performance integrated optical device, as compared to the region selective growth method to be described below, which forms core layers of a plurality of waveguide optical devices at once by single selective growth.
On the other hand, the region selective growth method is a technology of utilizing a region selective growth effect in metal organic chemical vapor deposition using an insulator mask, to thereby positionally control the bandgap energy and the thickness of a crystal layer in the substrate plane. According to this technology, core layers of a plurality of waveguide optical devices can be formed at once by single crystal growth. The region selective growth method is therefore widely used as a method of manufacturing a low-cost integrated optical device with a smaller number of crystal growths.
An exemplary integration technology from the viewpoint of how to connect mesa structures of waveguide optical devices together is an integration technology for a high-mesa optical waveguide and a low-mesa optical waveguide. As used herein, the high-mesa optical waveguide and the low-mesa optical waveguide are the names of a mesa optical waveguide structure grouped from the viewpoints of the mesa height and the positional relationship between the mesa and a core layer.
Specifically, the high-mesa optical waveguide is a mesa optical waveguide formed by processing a semiconductor multilayer structure including an upper clad layer, a core layer, and a lower clad layer by etching. The high-mesa optical waveguide has a feature that the core layer is positioned inside the mesa, and the mesa height is larger as compared to the low-mesa optical waveguide to be described below. Other features of the high-mesa optical waveguide include a larger light confinement effect and a small electric capacitance. The structure of the high-mesa optical waveguide is thus widely used for a Mach-Zehnder modulator and the like.
On the other hand, the low-mesa optical waveguide is a mesa optical waveguide formed by processing the upper clad layer of the semiconductor multilayer structure by etching. Unlike the high-mesa optical waveguide, the core layer is not etched in the low-mesa optical waveguide. In the low-mesa optical waveguide, the core layer is therefore positioned outside the lower side of the mesa, and the mesa height is smaller as compared to the above-mentioned high-mesa optical waveguide. The feature of the low-mesa optical waveguide is a small light confinement effect. The structure of the low-mesa optical waveguide is thus widely used for a semiconductor laser and the like.
As a known example of the conventional integration technology for the high-mesa optical waveguide and the low-mesa optical waveguide, an integrated optical device in which a semiconductor Mach-Zehnder modulator having a high-mesa optical waveguide structure and a semiconductor optical modulator having a low-mesa optical waveguide structure are monolithically-integrated in the same substrate plane is disclosed in Japanese Patent Application Laid-open No. 2008-66703. Another integrated optical device in which a wavelength converter having a low-mesa optical waveguide structure and a semiconductor Mach-Zehnder modulator having a high-mesa optical waveguide structure are monolithically-integrated is disclosed in Steven C. Nicoles, et al., “Integration Technologies for an 8×8 InP-Based Monolithic Tunable Optical Router with 40 Gb/s Line Rate Per Port”, Conference Proceedings of 22th International Conference on Indium Phosphide and Related Materials, 31 May-4 June, WeA3-1, pp. 160-163, 2010.